and Public Health
Review
Process Waters from Hydrothermal Carbonization of
Sludge: Characteristics and Possible
Valorization Pathways
Michela Langone1,* and Daniele Basso21 Laboratory of Technologies for the efficient use and management of water and wastewater, Italian National Agency for New Technologies, Energy and Sustainable Economic Development (ENEA), 00123 Roma, Italy 2 HBI S.r.l., 39100 Bolzano, Italy; d.basso@hbigroup.it
* Correspondence: michela.langone@enea.it
Received: 29 July 2020; Accepted: 7 September 2020; Published: 11 September 2020
Abstract: Hydrothermal carbonization (HTC) is an innovative process capable of converting wet biodegradable residues into value-added materials, such as hydrochar. HTC has been studied for decades, however, a lack of detailed information on the production and composition of the process water has been highlighted by several authors. In this paper the state of the art of the knowledge on this by-product is analyzed, with attention to HTC applied to municipal and agro-industrial anaerobic digestion digestate. The chemical and physical characteristics of the process water obtained at different HTC conditions are compared along with pH, color, organic matter, nutrients, heavy metals and toxic compounds. The possibility of recovering nutrients and other valorization pathways is analyzed and technical feasibility constraints are reported. Finally, the paper describes the main companies which are investing actively in proposing HTC technology towards improving an effective process water valorization.
Keywords:digestate; hydrothermal carbonization (HTC); HTC chemicals; process waters; sewage sludge
1. Introduction
Research in renewable energy production from bioresources, such as organic resources and waste, is constantly increasing in intensity and quality, pushed by the current rate of exploitation of fossil fuels and their related impacts and green house gases (GHGs) emissions on the environment. The use of anaerobic digestion (AD) for the energetic utilization of agro-industrial residues and even biodegradable municipal wastes is raising year after year (Figure1a). The main products of the AD are biogas and digestate. While biogas is a valuable source of energy, as it can be used in combined heat and power (CHP) generators or upgraded to biomethane for vehicles fuel and immission into the natural gas grid, after undergoing an upgrading process, digestate in some contexts could represent a problem to the environment or can sometimes encounter disposal-related issues.
(a)
(b)
Figure 1. Number of scientific publications containing as keywords: (a) “renewable energy and
waste/wastewater” and “anaerobic digestion”; (b) “hydrothermal carbonization (HTC)” and “hydrothermal carbonization (HTC) and sludge”, based on the NBCI bibliographic database from 2000 to 2018 [1].
Digestate is a heterogeneous solid–liquid by-product produced in large amounts as result of the AD process [2]. It contains a high proportion of mineral nitrogen (N), mainly in the form of ammonium, together with other macro- and micro-elements necessary for plant growth, which make it exploitable as an organic fertilizer. Furthermore, its organic residual content makes it suitable as organic amendment, improving the physical, chemical and biological properties of the soil [3]. According to the current European legislation, when AD is applied to pre-selected biowastes, digestate can be sent for direct soil application, reducing the needs of chemically produced fertilizers [4]. Often, various treatment options can be applied both to optimize transport and application conditions, such as solid/liquid separation, composting, drying, thermal concentration, physical–chemical treatment [5] and pelleting [6], as well as to treat it in order to reduce the nitrogen content [7]. However, the amount of digestate that can be applied to the field is
Figure 1. Number of scientific publications containing as keywords: (a) “renewable energy and waste/wastewater” and “anaerobic digestion”; (b) “hydrothermal carbonization (HTC)” and “hydrothermal carbonization (HTC) and sludge”, based on the NBCI bibliographic database from 2000 to 2018 [1].
Digestate is a heterogeneous solid–liquid by-product produced in large amounts as result of the AD process [2]. It contains a high proportion of mineral nitrogen (N), mainly in the form of ammonium, together with other macro- and micro-elements necessary for plant growth, which make it exploitable as an organic fertilizer. Furthermore, its organic residual content makes it suitable as organic amendment, improving the physical, chemical and biological properties of the soil [3]. According to the current European legislation, when AD is applied to pre-selected biowastes, digestate can be sent for direct soil application, reducing the needs of chemically produced fertilizers [4]. Often, various treatment options can be applied both to optimize transport and application conditions, such as solid/liquid separation, composting, drying, thermal concentration, physical–chemical treatment [5] and pelleting [6], as well as to treat it in order to reduce the nitrogen content [7]. However, the amount of digestate that can be applied to the field is limited according to its nutrient content, its quality and, in some countries,
according to the lack of fertile lands [4]. All these variables represent limiting factors of the application of digestate as organic fertilizer to soils. Moreover, depending on the size of the biogas plant, on the transportation costs and on the type of initial feedstocks, application of digestate on the field is not always economically feasible or allowed. In this sense, the importance of digestate management has been recognized by research and policy institutions, requiring a more sustainable approach to be adopted, mainly considering this waste as a feedstock for further bio-transformation [8] and thermal transformation [9] in a circular economy framework.
Recently, a relatively new technology, hydrothermal carbonization (HTC), has been proposed as a novel solution to treat biomass residuals, and among those digestates [10] producing a solid value-added product is hydrochar, which can be used as a high-quality alternative fuel or as a soil amender [11] and carbon sink [12]. Even if HTC has been known for about a century, only over the past two decades has it received greater attention in biomass conversion research (Figure1b). A wide range of biomass feedstocks, including cellulose [13], bamboo [14], loblolly pine [15], fruit waste [16], olive waste [17], tomato peel [18], agro-industrial waste [19], walnut shell and sunflower stem [20], microalgae [21,22], municipal solid food waste [23,24], distiller’s grains [25], animal manure [26,27], have been applied in HTC to gain fuels or materials. Interestingly, HTC has also been applied to municipal sewage sludge [28–31], anaerobically digested sewage sludge (ADSS) [32] and anaerobically digested biomasses [10].
HTC has attracted a great deal of interest primarily because it uses water as a reaction medium, which is a non-toxic, environmentally benign, and inexpensive reactant. This advantage is further increased when treating wet biomass, such as sludge and digestate, where water is inherently present. The benefits gained by carbonizing the digestate by HTC has been investigated by several research groups [33,34]. The integration of biogas and biochar promises several synergies, improving energy recovery from biomass wastes. However, large quantities of process waters are obtained as a by-product of the HTC process that have to be managed, treated and possibly valued [35].
HTC is a wet thermochemical process which involves the application of heat and pressure to convert raw material in the presence of water into a carbonaceous biofuel. A series of hydrolysis, condensation, decarboxylation and dehydration reactions occur during HTC (Figure2). Reaction temperatures are applied within a range of 180–250◦
C and pressures are maintained above the corresponding saturation pressure (10–50 bar) to ensure the liquid state of water [36]. Treatment times were reported to vary between a few minutes to several hours by many authors. In comparison to pyrolysis, HTC requires wet feedstock and therefore the digestate, which usually has a high moisture content of 54.3–98.5% [3], does not need to be dried prior to or during the process, saving a substantial amount of energy. Thus, the water contained in the raw digestate can be directly used as a reaction medium [37].
However, many studies have largely focused on the properties of the resulting solid product (hydrochar), energy recovery, hydrochar yields and its combustion characteristic. On the contrary, very few studies began to characterize the HTC process waters, oriented towards their valorization, changing them from wastes into products with a much greater value (chemicals, fuels, energy, as well as many other products beneficial for a local economy). This gap was highlighted by Owsianiak et al. [38] in their life cycle assessment (LCA) analysis, where it is shown that there are still considerable uncertainties about both the composition of HTC process waters and their potential valorization, recommending that technology developers measure the composition of process waters, especially with respect to possible application as fertilizer or for chemicals recovery. Thus, this manuscript presents the current state of the art for hydrothermal technologies in the digestate processing field for pretreatment and conversion of biomass and sewage sludge to chemical or fuel precursor, aiming to underline the possibility of valorizing the liquid by-product.
HTC PROCESS HTC PRODUCTS Biomass hydrolysis decarboxylation HTC gases Hydrochar Water HTC liquor Degraded biomass Organic acids Soluble organic compounds Other soluble compounds
Water Temperature Pressure
Figure 2. HTC process.
However, many studies have largely focused on the properties of the resulting solid product (hydrochar), energy recovery, hydrochar yields and its combustion characteristic. On the contrary, very few studies began to characterize the HTC process waters, oriented towards their valorization, changing them from wastes into products with a much greater value (chemicals, fuels, energy, as well as many other products beneficial for a local economy). This gap was highlighted by Owsianiak et al. [38] in their life cycle assessment (LCA) analysis, where it is shown that there are still considerable uncertainties about both the composition of HTC process waters and their potential valorization, recommending that technology developers measure the composition of process waters, especially with respect to possible application as fertilizer or for chemicals recovery. Thus, this manuscript presents the current state of the art for hydrothermal technologies in the digestate processing field for pretreatment and conversion of biomass and sewage sludge to chemical or fuel precursor, aiming to underline the possibility of valorizing the liquid by-product.
2. Digestate
Digestate is characterized by a high humidity content, commonly higher than 90%. The whole digestate composition depends on the type of input materials loaded into the biogas plant, as well as on the type of AD reactor [39]. Various types of feedstock have been reported in the literature, in particular, municipal primary sludge and waste-activated sludge [40], cattle manure [41], pig manure [42], solid organic waste [43], agricultural residues [44] and combinations of feedstocks, such as agricultural waste and dairy cow manure [45], sewage sludge and the organic fraction of municipal solid wastes [46], dairy cattle slurry and industrial meat-processing by-products [47], chicken manure and corn stover [48], etc. Table 1 contains indicative and non-exhaustive examples of the variability of digestate with respect to the initial feedstocks.
Municipal ADSS are characterized by a lower percentage of biodegradable carbon content as compared with digestate from other biomasses, which can be explained by the previous conversion of organic compounds into CO2 in the biological processes occurring during the wastewater
treatment line. Digested sludge from an agricultural biogas plant has a higher carbon content due to the characteristics of the raw biomass used as feedstock. Nitrogen concentrations in pig slurry co-digestion plants are quite similar, whereas in biogas plants which treat organic wastes, the nitrogen concentrations vary strongly. Lower nitrogen contents are present in digestate from cow manure co-digestion plants and municipal biogas plants. The reason for this is mostly the different
Figure 2.HTC process.
2. Digestate
Digestate is characterized by a high humidity content, commonly higher than 90%. The whole digestate composition depends on the type of input materials loaded into the biogas plant, as well as on the type of AD reactor [39]. Various types of feedstock have been reported in the literature, in particular, municipal primary sludge and waste-activated sludge [40], cattle manure [41], pig manure [42], solid organic waste [43], agricultural residues [44] and combinations of feedstocks, such as agricultural waste and dairy cow manure [45], sewage sludge and the organic fraction of municipal solid wastes [46], dairy cattle slurry and industrial meat-processing by-products [47], chicken manure and corn stover [48], etc. Table1contains indicative and non-exhaustive examples of the variability of digestate with respect to the initial feedstocks.
Municipal ADSS are characterized by a lower percentage of biodegradable carbon content as compared with digestate from other biomasses, which can be explained by the previous conversion of organic compounds into CO2in the biological processes occurring during the wastewater treatment line.
Digested sludge from an agricultural biogas plant has a higher carbon content due to the characteristics of the raw biomass used as feedstock. Nitrogen concentrations in pig slurry co-digestion plants are quite similar, whereas in biogas plants which treat organic wastes, the nitrogen concentrations vary strongly. Lower nitrogen contents are present in digestate from cow manure co-digestion plants and municipal biogas plants. The reason for this is mostly the different nitrogen concentrations in the corresponding substrates. Furthermore, the process design, e.g., the amount of fresh water and recirculation effluent in use, may influence the total nitrogen concentrations.
The analysis of the results showed that, independently from the initial feedstock, digestate has a relatively high metals content, in particular Fe, Al and Ca, which could interfere with phosphorous recovery; this represents one interesting and viable possibility to valorize this feedstock through HTC. Iron concentrations in municipal sewage sludge are high because, for example, it is added in sewage pumping stations to control odors and corrosion, and further iron or aluminum salts are added in municipal wastewater treatment to remove phosphate from wastewater with the intention to prevent eutrophication of the surface [49]. Instead, in agro-industrial biogas plants the use of micronutrients is being consolidated (i.e., Mo, Ni, Co, Se, Fe) to improve biogas production [50].
In order to optimize its management, raw digestate commonly undergoes to mechanical solid–liquid separation by means of a screw press or a decanter centrifuge, where a fiber/solid fraction and a liquid fraction are separated. The solid fraction of digestate usually displays significantly higher concentrations of phosphorus, organic nitrogen, organic carbon, whereas liquid fractions tends to have higher concentrations of available nitrogen and orthophosphate. Typical ranges for the distribution of the main constituents between the fiber/solid and the liquor are provided by several studies [51,52]. It is worth noting that the HTC process can be applied to the whole digestate or to the dewatered solid digestate.
Table 1.Digestate characteristics. Reference Alburquerque et al.
(2012) [53] Alburquerque et al. (2012) [53] Alburquerque et al. (2012) [53] Alburquerque et al. (2012) [53] Uysal et al. (2010) [54]
aPeng and Pivato, (2019) [55]bTampio et al.
(2016) [56] Digestate Pig Slurry and
Energy-Crop Residues
Pig Slurry and Animal By-Products
Cattle Manure and Glycerin
Cattle Manure and Agro-Industrial
Residues
Municipal Sewage Sludge
Organic Solid Waste
pH 7.80–7.90 7.86–8.20 5.64–7.35 7.50–7.90 7.6 7.60–8.30a
EC [dS m−1] 23.3–26.0 21.1–30.8 5.20–14.5 8.7–25.7 -
-TS [g L−1] 28.3–43.9 19.5–29.5 17.6–72.9 17.6–90.1 25.3 ± 0.2 7.2–78.8a
TOC [g L−1] 8.3–14.7a 5.8–8.4 8.3–42.8 5.8–33.7 -
-COD [g L−1] 3.7–4.3 1.2–3.5 8.2–27.6 1.0–5.4 25.8 ± 1.9 21.8–100.3b
BOD5[g L−1] 4.0–6.5 2.2–6.2 10.6–52.5 1.2–5.9 (as SCOD)0.4 ± 0.03 7.3–15.4 (as SCOD)b
TN [g L−1] 3.4–3.6 2.9–4.9 0.6–2.3 1.4–4.0 1.0 ± 0.02 4.7–8.7b NH4+-N [g L−1] 2.6–2.9 2.2–3.5 0.4 – 1.0 0.8–2.4 0.9 ± 0.01 1.7–27.5 a 1.7–4.5b TP [g L−1] 1.2–1.2 0.2–0.8 0.8–1.8 0.2–0.8 0.39 ± 0.003 -PO4−-P [g L−1] - - - - 0.021 ± 0.0 -K [g L−1] 2.7–3.1 2.0–3.1 0.8–1.8 1.1–3.1 0.074 ± 0.005 -Al [mg L−1] - - - - 91 ± 10 -S [mg L−1] 367–417 219–680 48–265 113–457 - -Ca [mg L−1] 1863–1993 218–828 192–1753 1008–4026 1049 ± 57 -Mg [mg L−1] 633–721 67–365 79–333 257–698 194 ± 1.5 -Na [mg L−1] 666–699 696–995 66–1842 276–746 175 ± 8.2 -Cl [mg L−1] 1495–1613 1598–2120 448–685 452–1418 - -Fe [mg L−1] 143–224 22–63 95–165 30–301 318 ± 32.5 -Mn [mg L−1] 23–31 2.9–15.4 3.2–17.1 6.0–27.5 3.6 ± 0.1 -Zn [mg L−1] 45.9–62.5 34.7–140.2 10.6–28.3 7.7–27.7 51 ± 5.4 56–300a Cu [mg L−1] 7.0–8.4 4.0–15.1 1.4–13.0 2.8–10.8 4.0 ± 0.1 14–80a B [mg L−1] 2.7–3.2 2.2–3.1 1.3–4.8 1.7–3.5 -
-SCOD= soluble chemical oxygen demand.aPeng and Pivato, (2019) [55],bTampio et al. (2016) [56].
3. Hydrothermal Carbonization
HTC is a valid strategy for treatment and valorization of biomass residuals, especially those with a high water content (up to 80%) such as digestate. The HTC process mainly results in three types of products: a solid hydrochar and aqueous and gaseous by-products (Figure3).
During HTC organic substrates in sealed containers are mixed with water under mild temperature conditions (180–250◦C) and pressure slightly higher than the water saturation pressure to ensure that water remains in its liquid state. Under this range of temperature, the water is in subcritical conditions and acts as an effective solvent, catalyst and reactant for hydrolytic conversion and extractions [57] from wet biomasses and waste streams. Typical chemical reactions occurring during HTC are hydrolysis, dehydration, decarboxylation, aromatization, condensation and polymerization [11,58]. High moisture content favored decarboxylation and hydrolysis reactions [59]. Mainly, the carbon content of the feedstock increases, achieving a higher calorific value [60] and producing a solid value-added product, namely hydrochar.
Int. J. Environ. Res. Public Health 2020, 17, 6618 6 of 31 3. Hydrothermal Carbonization
HTC is a valid strategy for treatment and valorization of biomass residuals, especially those with a high water content (up to 80%) such as digestate. The HTC process mainly results in three types of products: a solid hydrochar and aqueous and gaseous by-products (Figure 3).
Figure 3. HTC by-products.
During HTC organic substrates in sealed containers are mixed with water under mild temperature conditions (180–250 °C) and pressure slightly higher than the water saturation pressure to ensure that water remains in its liquid state. Under this range of temperature, the water is in subcritical conditions and acts as an effective solvent, catalyst and reactant for hydrolytic conversion and extractions [57] from wet biomasses and waste streams. Typical chemical reactions occurring during HTC are hydrolysis, dehydration, decarboxylation, aromatization, condensation and polymerization [11,58]. High moisture content favored decarboxylation and hydrolysis reactions [59]. Mainly, the carbon content of the feedstock increases, achieving a higher calorific value [60] and producing a solid value-added product, namely hydrochar.
Hydrochar can be defined as a homogenized, carbon-rich and energy-rich solid fuel, biologically sterilized due to thermal treatment, with a similar energy density as lignite, which contains most of the organic compounds originally present within the feedstock. Its characterization, production yield and higher heating value (HHV) depend on feedstock type and HTC process parameters [11,61]. Its utilization as a stable energy-rich solid fuel in combustion processes is widely recognized [28]. However, in recent years, other uses of the hydrochar have emerged: it can be exploited as a soil amendment and nutrient source, as low-cost adsorbent for contaminants in aqueous solutions, for the generation of nano-structured materials [62], as precursor for activated carbons [63], as a catalyst [64], as well as for CO2 sorption and sequestration [65]. Moreover,
hydrochar has been applied in the AD process to promote the biomethane production, by acting both as support for bacteria colonies, conductor for electron transfer among species, sorbent for indirect inhibitors, and reactant in chars labile carbon methanization [66,67]. Further, its pelletization has been studied by Wang et al. [24].
HTC treatment also produces a process water, as a result of the amount of water initially present within the HTC reactor. Process waters are a liquor rich in dissolved organic components and inorganic salts [58]. Like the solid product, process waters are highly dependent on feedstock type and HTC process parameters [35]. They can be recycled to the HTC process or further treated. However, in the contest of maximizing energy and resource recovery, HTC process waters need to be valorized. Finally, HTC also produces a very small amount of gas, about 1–3% of the mass of the raw material, which is mainly composed of CO2 with traces of CO [23]. Depending on HTC process
conditions, oily molecules (water-insoluble) may also be generated in hydrothermal processes
Figure 3.HTC by-products.
Hydrochar can be defined as a homogenized, carbon-rich and energy-rich solid fuel, biologically sterilized due to thermal treatment, with a similar energy density as lignite, which contains most of the organic compounds originally present within the feedstock. Its characterization, production yield and higher heating value (HHV) depend on feedstock type and HTC process parameters [11,61]. Its utilization as a stable energy-rich solid fuel in combustion processes is widely recognized [28]. However, in recent years, other uses of the hydrochar have emerged: it can be exploited as a soil amendment and nutrient source, as low-cost adsorbent for contaminants in aqueous solutions, for the generation of nano-structured materials [62], as precursor for activated carbons [63], as a catalyst [64], as well as for CO2 sorption and sequestration [65]. Moreover, hydrochar has been applied in the
AD process to promote the biomethane production, by acting both as support for bacteria colonies, conductor for electron transfer among species, sorbent for indirect inhibitors, and reactant in chars labile carbon methanization [66,67]. Further, its pelletization has been studied by Wang et al. [24].
HTC treatment also produces a process water, as a result of the amount of water initially present within the HTC reactor. Process waters are a liquor rich in dissolved organic components and inorganic salts [58]. Like the solid product, process waters are highly dependent on feedstock type and HTC process parameters [35]. They can be recycled to the HTC process or further treated. However, in the contest of maximizing energy and resource recovery, HTC process waters need to be valorized. Finally, HTC also produces a very small amount of gas, about 1–3% of the mass of the raw material, which is mainly composed of CO2with traces of CO [23]. Depending on HTC process conditions,
oily molecules (water-insoluble) may also be generated in hydrothermal processes [68,69]. Furthermore, on the industrial scale, when steam is used as a heating medium, a condensate rich in volatile organic compounds can be obtained from HTC [70].
The formation, the yields and the composition of solid, liquid and gaseous by-products depend on the biomass being processed, as well as on the processing HTC conditions, such as residence time and temperature [34] and biomass-to-water ratio (b/w) [59].
Usually, reaction time can vary from a few minutes up to several hours (12–20 h), even if most of the reactions seem to last within the first reaction hour [71]. However, it has been observed that both time and temperature influence product characteristics [61], the latter being the most influencing process parameter [58].
Further, the addition of some additives, such as acids, bases and salts, can affect the characteristics of products formed, having an effect on the degree of carbonization (described by the H/C ratio), the HHV and the extraction of certain elements. Moreover, additives may act as catalysts, thus reducing the HTC reaction temperatures and pressures [72,73]. The water pH has a significant impact on the hydrochar yield, its HHV and ash content [73], and, as a consequence, the addition of selected salts can increase the HHV of the obtained hydrochar [74]. The most frequently reported mineral acids are hydrochloric and sulfuric acid [72,73], while the organic acids used are acetic and citric acid [73,75,76]. Stemann et al. [77] showed that by process water recirculation, organic acids in the liquid phase catalyze HTC reactions. NaOH, Ca(OH)2are used as bases [72,78], while NaCl and CaCl2[72] and Ca
propionate, Ca acetate, Mg acetate, Ca lactate, Li chloride, Ca chloride and Ca formate are commonly used as salts [74]. CaO additive has been employed in near-critical water to act as CO2absorber via
carbonation [59], while zeolite has been used as a catalyst in the HTC process, increasing the energy and carbon recovery in hydrochar from digestate [79].
4. Hydrothermal Carbonization of Digestate
HTC has been applied for a variety of feedstocks, including digestate produced by means of AD of different feedstocks (Table2). Digestate can be treated directly in the HTC process as wet feedstock, or it can be treated after a solid/liquid separation process. In some studies, despite the advantage of HTC in treating wet feedstock, the digestate was dried in an oven for two–three days at 60◦C to ensure a homogenous dry matter content and consistent carbon content [80].
Examples of HTC applications will be considered here to illustrate the increasing importance of the processes in sludge and digestate treatment.
Table 2.Hydrothermal carbonization (HTC) process applied to digestate.
HTC Feedstock Laboratory Treatment Prior
to HTC Reactor Volume HTC Conditions Studied Products/Characteristics Reference
ADSS - 160 mL 250◦
C, 20 h Process waters
Hydrochar Berge et al. (2011) [23]
ADSS - 500 mL 160, 220, 250◦C, 30 min Process waters
Hydrochar Aragón-briceño et al. (2017) [34]
ADSS - 200 mL 120–240 ◦ C, 1–60 min Process waters Hydrochar MAP precipitation Yu et al. (2017) [81]
Dewatered ADSS (solid) Water dilution
Use of CaO additive 1000 mL
200 to 380◦ C, 20 min 500 rpm Process waters Hydrochar MAP precipitation He et al. (2015) [59]
Dewatered ADSS (solid)
Water dilution in order to obtain a TS content of 20% Use of citric acid as catalyst
25.0 L
205◦C, 7 h pH regulation by acetic acid and
sodium hydroxide
Process waters Hydrochar Dewaterability
Escala et al. (2013) [82]
Dewatered ADSS (solid) Pre-dried at 105
◦ C for 12 h Water dilution 1000 mL 200 to 380◦ C, 20 min 500 rpm Process waters Hydrochar Ammonia stripping He et al. (2015) [83]
Dewatered ADSS (solid) - 200 mL 160, 200, 240◦C, 4, 8, 12 h
Process waters Hydrochar N distribution
Shen et al. (2018) [84]
Dewatered ADSS (solid) Water dilution
Addition of Cd 200 mL 200, 280 ◦ C, 1 h Process waters Hydrochar P distibution Shi et al. (2014) [85]
Dewatered ADSS (solid) Water dilution
Addition of Cr, Ni, Cu, Zn, Cd, Pb 200 mL 170, 200. 280
◦ C, 1 h Process waters Hydrochar P and HM distribution Shi et al. (2013) [86]
Dewatered ADSS (solid) - 125 mL 200
◦
C,
4, 6, 8, 10, 12 h Hydrochar He et al. (2013) [28]
ADSS Water dilution 1000 mL 180–200
◦
C, 30 min 200 rpm
Hydrochar
Dewaterability Kim et al. (2014) [32]
ADSS Water dilution 200 mL 200◦C, 24 h Hydrochar Alatalo et al. (2013) [87]
Dewatered ADSS (solid) Pre-dried at 105
◦
C for 24 h
Triturated 500 mL 200, 230, 260
◦
C, 2 h Hydrochar
P evolution Wang et al. (2017) [78]
Dewatered ADSS (solid) Water dilution 20 mL 225◦
C, 4–16 h Hydrochar
Table 2. Cont.
HTC Feedstock Laboratory Treatment Prior to
HTC Reactor Volume HTC Conditions Studied Products/Characteristics Reference
ADSS 3.0 m3 200
◦
C, 6 h pH regulation by citric acid
Process waters HTC+ AD Wirth et al. (2015) [89] AGS - 200 mL 160, 200, 240◦ C, 1 h Process waters Hydrochar AD+HTC Yu et al. (2018) [90] ADSS 3.0 m3 200 ◦ C, 6 h
pH regulation by citric acid Condensate Wirth and Reza (2016) [70] Anaerobically digested wheat
straw (thermophilic digestion)
Pre-dried at 60◦C for 72 h Water dilution in order to obtain a carbon concentration of 26.6 g L−1
1000 mL 190, 210, 230, 250
◦
C,
1, 2.5, 4 h Hydrochar Funke et al. (2013) [33]
Anaerobically digested maize silage (thermophilic digestion)
Water dilution in order to obtain a
carbon concentration of 42.3 g L−1 1000 mL
190, 230, 270◦C, 2, 6, 10 h
90 rpm pH regulation by citric acid
(pH 3, 5, 7)
Hydrochar Mumme et al. (2011) [10]
Anaerobically digested wheat straw (thermophilic digestion)
Water dilution in order to obtain a
TS content of 10% 1000 mL
230◦
C, 6 h
90 rev/min Hydrochar Mumme et al. (2014) [66] Dried anaerobically digested cow
manure and maize (mass ratio of 4:3 as feedstock) and zeolite
Pre-dried at 105◦C for 24 h Cut to particle size below 1 mm
Water dilution
1000 mL
190, 230, 270◦C, 2 h 90 rev/min Addition of zeolite
Hydrochar–zeolite composite Mumme et al. (2015) [79]
Anaerobically digested
agro-industrial biomass Pre-dried at 60
◦
C for 48 h 75 mL 250◦
C, 1 h Process waters
Hydrochar Ekpo et al. (2016) [91] Anaerobically digested wheat
straw (thermophilic digestion)
Pre-dried at 60◦
C for 48 h Water dilution in order to obtain a carbon concentration of 26.7 g L−1
125 mL 190, 230, 250, 270◦
C, 6.0 h Process waters Becker et al. (2014) [80]
Anaerobically digested corn silage
-Full-scale plant located at Karlsruhe, Germany 220◦ C, 6.0 h Process waters Valorization process waters
througth AD
Wirth and Mumme (2014) [92]
Dewatered anaerobically digested
algal biomass Water dilution 300 mL 200
◦
C, 1 h Process waters
HTC+ AD system Nuchdang et al. (2018) [93] Dewatered anaerobically digested
agro-industrial biomass 25.0 L 180
◦
C, 4 h Process waters
Hydrochar Oliveira et al. (2013) [26] Dewatered anaerobically digested
municipal solid waste - 100 mL
200, 250, 300◦C 0.5, 2 h 90 rpm
Process waters,
Hydrochar Reza et al. (2016) [94]
Anaerobically digested corn silage Full-scale plant, Germany 180◦C, 8–10 h
Process water Hydrochar
Toxicity
Bargmann et al. (2013) [95]
As far as municipal anaerobically digested sewage sludge (ADSS) is concerned, both the HTC solid and liquid by-products have been characterized, following the fate of carbon [23], nitrogen and phosphorus [33,81,96] in the product streams. The speciation of phosphorus in sludge and hydrochar has been thoroughly studied [76,94]. Kim et al. [32] showed that HTC can be used to successfully treat ADSS, producing a carbonaceous solid product, hydrochar, with a lower moisture content and HHV than the raw digestate, making it a viable alternative to fossil solid fuels. The fuel characteristics and combustion behavior of this type of hydrochar were both studied by He et al. [28], while its adsorption behavior was investigated by Alatalo et al. [87]. Aragón-briceño et al. [34] studied the valorization of the HTC process waters obtained from an ADSS, showing that HTC can increase the biomethane potential production of the whole HTC by-product (hydrochar+ processed water) and in process waters alone. Wirth et al. [89] studied the influence of AD temperature and organic loading rate on the continuous anaerobic treatment of process waters from HTC of ADSS. Valorization of HTC process waters from ADSS through AD was further evaluated by Yu et al. [90]. HTC condensate from steam-derived HTC of ADSS was further characterized and valorized by AD in the research conducted by Wirth and Reza [70]. The improvement of digestate dewaterability after an HTC process has also been studied [32,82]. Kim et al. [32] showed that the HTC process breaks up the physical structure of the sludge, converting the bound water to free water within the sludge. The use of catalysts for the HTC process was investigated by Escala et al. [82], who used citric acid obtaining a hydrochar with a slightly higher C content compared to the samples without a catalyst and a lower HTC reaction time. The effect of calcium oxide on the evolution profile and characteristics of gas, solid and liquid by-products of HTC from ADSS was reported by He et al. [59]. The phosphorous and heavy metal distribution in the by-products of dewatered ADSS was investigated by several authors [83,84]. In order to reduce the considerable emissions of N-containing pollutants from combustion of the sewage sludge-derived solid fuel, an integrated system of hydrothermal deamination and air stripping from HTC water processing was developed to effectively remove and recover nitrogen from dewatered sewage sludge [59]. Similarly, Shen et al. [84] proposed a combined HTC and CO2gasification process
that could be coupled with a struvite precipitation or air-stripping treatment of HTC process waters in order to recover nitrogen. Recently, Yu et al. [81] proposed HTC pretreatment of ADSS coupled with MgNH4PO3· 6 H2O (MAP) crystallization for the simultaneous recovery of phosphorus and nitrogen.
With reference to the HTC of digestate from agro-industrial waste, some experiences are reported in the literature. Funke et al. [33] studied a combined system that combines AD of wheat straw with HTC of digestate, doubling the energy recovery obtained by the single AD process. Indeed, about 50% of the AD total energy input can be recovered in the hydrochar, which represents a valuable fuel with an estimated HHV (on a dry matter basis) of 31.5 MJ kg−1. Similarly, Oliveira et al. [26] applied HTC to digestate of agricultural residues, producing a solid fuel (hydrochar) comparable to brown coal, which could be used to complement farm-based biogas plants [26]. Mumme et al. [10] applied HTC at an anaerobically thermophilic digested maize silage, showing that relatively mild conditions (190◦C, 2 h) are suitable for producing hydrochar which is potentially interesting as an alternative fuel or soil conditioner. Interestingly, Mumme et al. [79] studied the catalytic effects of zeolite on carbonization
of agro-industrial digestate, producing a hydrochar−zeolite composite with potential applications ranging from energetic use to soil amendment and additive in bioprocesses (e.g., growing media). The use of hydrochar obtained from anaerobically thermophilic wheat straw digestate as adsorbent in AD processes was studied by Mumme et al. [66], who showed that the addition of hydrochar increased the methane yield by 32% due to the ability to catalyze AD by mitigation of mild ammonia inhibition, support of growth of archea and methanization of the hydrochar’s labile carbon.
HTC process water composition from an agro-industrial digestate was analyzed by Ekpo et al. [91]. Becker et al. [80] applied the HTC to wheat straw digestate and studied the composition of process waters, in terms of major organic components. Oliveira et al. [26] proposed that the process waters can be recycled in the biogas plant, reducing their environmental impacts and increasing the energy efficiency of the whole process. The combined system, with AD as primary treatment and HTC
as post-treatment applied to the solid digestate, has also been investigated to treat microalgae by Nuchdang et al. [93], proving that applying HTC to digestate and recycling the HTC process water to the primary AD would increase methane production by 40%. The possibility to valorize the HTC process water through AD has also been considered for HTC liquor produced from anaerobically digested corn silage [92].
Recently, digestate from organic fraction of municipal solid waste (OFMSW) was treated by HTC by Reza et al. [94], obtaining an energy–dense solid hydrochar and a process water with mainly organic acids and amino acids. Zhou et al. [97] underlined in their study the synergistic effect of the
combination of AD and HTC, in order to evaluate a sustainable treatment for food waste in China. Life cycle assessment (LCA) analyses of HTC technology have been carried out in order to identify the processes configuration with the largest potential, and to drive the environmentally conscious design of future HTC. Berge et al. [23] showed how the impacts of HTC of food waste and combustion of the resulting hydrochar in a power plant depend mainly on process waters treatment and valorization and the type of energy that is substituted. Similarly, Owsianiak et al. [38] using LCA, showed that HTC of green waste, food waste, organic fraction of municipal solid waste (MSW), and digestate followed by energy recovery, when hydrochar is used as solid fuel, is a potential technology to treat biowaste, underlining that both process waters and hydrochar compositions are important parameters influencing environmental performance. For example, across the life cycle impact, emissions of metallic elements from HTC process water discharge drove toxic impacts on human health and ecosystems, whereas the use of hydrochar as fuel, substituting energy derived from fossil sources, such as anthracite or lignite, resulted in the best environmental performance.
5. Hydrothermal Carbonization Process Water Characterization
The yields and the composition of the HTC process waters depend on the biomass being processed as well as on the HTC processing conditions, mainly temperature, reaction time, and ratio of dry biomass to water (b/w). The yield of process waters was found to be in the range of 25–55 wt.% by He et al. [59], who performed HTC on dewatered anaerobically digested sewage sludge using HTC temperature higher than 200◦
C. The yield tends to increase when increasing both reaction temperature and the initial moisture content. This large range of values was attributed to the dissolution and decomposition of organic matter during the HTC process. Liquid yields of 17% and 30% were reported by Ekpo et al. [91], when treating agro-industrial digestate at 170 and 250◦C, respectively, as results of a combination of solubilization of inorganics and an increase in the production of soluble organic hydrocarbons.
The management of HTC process waters needs to be evaluated according to its quality. In general, HTC process waters contain high concentrations of organic matter as indicated by TOC (Total Organic Carbon), soluble COD (Chemical Oxygen Demand) and BOD5(Biochemical Oxygen Demand) values,
and the relative abundance of nutrients (N, P, K) [35]. Table3summarizes the main findings reported in the literature for HTC process waters from municipal sewage sludge digestate and agro-industrial digestate, respectively.
Table 3.Hydrothermal carbonization (HTC) process waters compositions. References Raw Material andHTC Conditions Yield pH TOC
Soluble
COD VFAs
Acetic
Acid TS Total N NH4+-N Total
Soluble P Ortho-P Total K Phenols Others C H N S O - mgC L−1 mg L−1 mgCOD
L−1 mg L−1 % mgN L−1 mgN L−1 mgP L−1 mgP L−1 mgK L−1 % % % % %
Berge et al. (2011) [23] ADSS HTC 250◦ C, 20 h - 8.0 4000 10000 YES1 YES1 Aragón-briceño et al. (2017) [34] ADSS 7.8 461.6 1843 4.8 4.5 1493 1344 91.3 80.1 30.5 4.4 10.2 0.7 54.1 HTC 160◦ C, 30 min 9.1 4686 12,642 191.1 2066 1258 94.0 53.9 45.8 6.8 11.1 1.9 34.5 HTC 220◦ C, 30 min 7.1 4584 12,992 406.0 2191 1704 72.6 59.8 49.2 6.3 12.3 2.4 29.8 HTC 250◦ C, 30 min 8.1 4879 12,164 715.7 2354 1685 103.8 56.8 68.0 6.6 6.6 1.8 10.9 He et al. (2015) [59] Dewatered ADSS 17.5 HTC 200◦ C, 20 min 87 (%vol) 8.6 24,070 63,900 12,000 4020 246 YES1 HTC 280◦ C, 20 min 98 (%vol) 8.4 16,000 40,000 10,100 6400 191 HTC 380◦ C, 20 min 95(%vol) 8.1 12,510 30,400 10,000 7980 89 HTC 380◦ C, 20min, CaO 10.0 18,630 55,800 12,000 8700 21 Escala et al. (2013) [82] Dewatered ADSS 6.9–7.4 23.9 HTC 205◦ C, 7h, Ca 7.0 53,000 2590 2047 14.3 11.5 666 HTC 205◦ C, 7h 6.9 40,600 2710 2153 17.8 4.8 633 Shi et al. (2014) [85] Dewatered ADSS 6.4 15 3150 3900 HTC 200◦C, 1h 0.2%+ HTC 280◦ C, 1h 0.6%+ Yu et al. (2017) [81] ADSS 6.3 1800 1.8 300 370 HTC 160◦ C, 30 min 6.0 4000 400 480 HTC 200◦ C, 30 min 5.7 5000 450 570 HTC 240◦ C, 30 min 5.5 6000 490 400 Shi et al. (2013) [86] Dewatered ADSS 6.4 14.5 HTC 170◦ C, 1 h 7.6 2357 12.5 HTC 200◦ C, 1h 8.5 2586 15.8 HTC 280◦C, 1 h 9.2 3566 30.4
Wirth et al. (2015) [89] ADSS HTC 200◦ C, 6 h, pH regulation 4.7 13,400 34300 2060 3.4 2800 1000 YES2 Yu et al., (2018) [90] AGS 6.8 1118 100 0 9.5 HTC 160◦ C, 1 h 6.0 15,611 454 300 HTC 200◦ C, 1 h 5.8 1100 900 HTC 240◦ C, 1 h 5.6 2557 2000 Ekpo et al. (2016) [91] Agro-industrial digestate HTC 250◦C, 1 h 30 (wt%) 7.7 62,350 18,610 10,235 840 2340
Table 3. Cont. References Raw Material andHTC Conditions Yield pH TOC
Soluble
COD VFAs
Acetic
Acid TS Total N NH4+-N Total
Soluble P Ortho-P Total K Phenols Others C H N S O - mgC L−1 mg L−1 mgCOD
L−1 mg L−1 % mgN L−1 mgN L−1 mgP L−1 mgP L−1 mgK L−1 % % % % %
Becker et al. (2014) [80]
Wheat straw digestate HTC 190◦ C, 6.0 h 4.0 5800 1000 YES3 YES3 HTC 230◦ C, 6.0 h 4.0 9000 1000 HTC 250◦ C, 6.0 h 4.0 7800 1250 HTC 270◦ C, 6.0 h 4.0 9500 1200
Wirth and Mumme (2014) [92]
Corn silage digestate HTC 220◦ C, 6.0 h 3.88 15,660 41,350 5260 2.8 685 229 197 290 Nuchdang et al. (2018) [93] Microalgae digestate 1926 0.9 HTC 200◦C, 1.0 h 8204 0.9 Reza et al. (2016) [94] MSW digestate 8.1 23 HTC 200◦ C, 30 min 8.2 HTC 200◦ C, 2.0 h 8.3 Bargmann et al. (2013) [95]
Corn silage digestate HTC 180◦
C, 9.0 h 5.7 83.3 20.3 328
11-Methyl-4-[nitromethyl]-4-piperidinol; 1-Methyldodecylamine; 1-Phenethyl-piperidin-4-ol; 1-Propanol, 2-amino-; 2,5-Pyrrolidinedione, 1-ethyl-; 2,5-Pyrrolidinedione, 1-methyl-; 2-Butanamine, (S)-; 2-Cyclopenten-1-one, 2,3-dimethyl-; 2-Cyclopenten-1-one, 2-methyl-; 2-Heptanamine, 5-methyl-; 2-Propanamine; 3-Aminopyridine; 3-Buten-2-one, 3-methyl-, dimethylhydrazone; 3-Cyclohexene-1-carboxaldehyde, 4-methyl-; 4-Fluorohistamine; Acetic acid; Benzoic acid, 2,4-dihydroxy-, (3-diethylamino-1methyl)propyl ester; Dimethylamine; dl-Alanine; Formic acid phenyl ester; Hydrogen chloride; Methylpent-4-enylamine; Phenethylamine, p-methoxy-.alpha.-methyl-, (.+/-.)-; Phenol; Phenol, 4-methyl-; Pyrazole, 1-methyl-4-nitro-; Tetrahydro-4H-pyran-4-ol. 2Furfural, 5-HMF, phenol, cresol, catechol, and resorcinol. 3Furfural (50–500 mg L−1), 5-HMF (0–100 mg L−1), phenolic compounds (2-methoxyphenol) (10–300 mg L−1), 2-Methylbenzofuran (2–70 mg L−1)+, expressed as redistribution of phosphorus in process waters (%). TOC= Total Organic Carbon COD = Chemical Oxygen Demand VFA= Volatile Fatty Acids NH4+-N= ammonium nitrogen.
5.1. pH and Color
The pH of HTC process waters from biomass is usually around 4.5 or below [35], except when treating ADSS, for which the pH is generally alkaline [23]. This is mainly due to the high buffering
capacity of the digestate. However, lower pH values of process waters obtained from dewatered ADSS were reported by Yu et al. [81], in the range of 5.6–6.8. HTC process waters from MSW digestate has a pH of around 8.0, while pH values in the range of acidity have been reported for process waters obtained from digestate of corn silage [92] and wheat straw [80], probably due to a lower buffering capacity of initial biomass. Ekpo et al. [91] reported pH values of HTC process waters from agro-industrial digestate close to 8.0, due to the presence of manure as feedstock of a biogas plant, which gives a higher buffering capacity. Furthermore, in process waters obtained from HTC of ADSS, Aragón-briceño et al. [34] showed that pH can be influenced by the HTC temperature, due to the presence of volatile fatty acids (VFAs), amino acids and ammonia nitrogen which are generated during the HTC treatment. The authors reported higher pH values at lower HTC temperatures (pH of 9.15 at 160◦
C vs. 8.08 at 250◦
C).
Process liquids from HTC are dark in color [94], a yellowish to dark brownish liquid [59]. Recently, Xu and Jiang [69], treating urban sewage sludge, reported that the color can be used as a crucial index to evaluate the purification of the process water, as it can be associated with the dissolution and decomposition of organic matter during HTC. It becomes lighter from black-brown to yellowish-brown with increasing temperature from 180 to 300◦C, respectively, while the content of organic matter decreases. Similar results were reported by He et al. [59] for dewatered ADSS.
5.2. Organic Compounds
Several organic compounds have been detected in the HTC process waters as a result of thermal degradation of the feedstock [23]. Table4summarizes compounds identified in the HTC process water from HTC of ADSS and gives an overview of their possible main applications in the chemical industry.
HTC process waters from municipal ADSS could contain up to 30% of the initial carbon (C) present in the feedstock [23]. A carbon fraction of 17% has been reported in the HTC process waters from agro-industrial digestate [26]. However, the C content distribution depends on both the initial characteristics of sludge and on the HTC process conditions. Indeed, using an anaerobic granular sludge (AGS), Yu et al. [90] reported a higher C content in the HTC process waters, from 37% at 160◦C to 62% at 240◦C, due to the high (volatile solid) content in the AGS as compared with a suspended sludge.
Both TOC and soluble COD (SCOD) concentrations in HTC process waters from ADSS can vary in a wide range from 4000 mg L−1[23] to 24,000 mg L−1[34], and from 10,000 mg L−1[23] to 64,000 mg L−1[59], respectively. HTC process temperature plays an important role in determining C content in process waters. Aragón-briceño et al. [34] reported that the concentration of SCOD in the HTC process waters from ADSS increased 7-fold, increasing from 1843 in the raw digestate to 12,642 mg L−1 after 160◦C treatment, to 12,992 mg L−1 after 220◦C treatment, and decreasing to 12,164 mg L−1after 250◦C treatment. A similar trend was observed for TOC that increased 10-fold after HTC treatment, from 461 to 4879 mg L−1. Similarly, for temperatures below 200◦C, treating a mixture of primary and secondary sludge, Qiao et al. [98] reported that SCOD of process water increased with increasing temperature, from 120 to 190◦C. However, after more than 30 min of heating, there was only a small increase in SCOD concentration. Up 240◦C, Yu et al. [81] reported a steady increase of TOC with the increase in the process temperature treating an ADSS. For temperatures higher than 200◦C, in their study, He et al. [59] showed that TOC and COD of process waters from ADSS decreased from 24.07 and 63.9 g L−1to 12.51 and 30.4 g L−1, respectively, by increasing the temperature from 200 to 380◦
C, while inorganic carbon (IC) was accumulated from 0.48 to 3.67 g L−1. This suggests that for
temperature higher than 200◦C, the gasification process became faster than hydrolysis and dissolution of organic substances, while carbon underwent gradual mineralization.
Table 4.Compounds identified in the HTC process water from HTC of anaerobically digested sewage sludge (ADSS) and their main possible application. Adapted from Berge et al. [23].
Compound Application
1-Methyl-4-[nitromethyl]-4-piperidinol Production of antitumor agents and products involved in the treatment of cardiovascular diseases
1-Methyldodecylamine Preparation of N,N,N,N,N,N- trimethyldodecylammonium bromide
1-Phenethyl-piperidin-4-ol
-1-Propanol, 2-amino- Organic syntheses (e.g, Schiff base ligands)
2,5-Pyrrolidinedione, 1-ethyl- Organic syntheses
2,5-Pyrrolidinedione, 1-methyl- Organic syntheses, as well as in some industrial silver-plating processes
2-Butanamine, (S) Production of some pesticides
2-Cyclopenten-1-one, 2,3-dimethyl-
-2-Cyclopenten-1-one, 2-methyl-
-2-Heptanamine, 5-methyl-
-2-Propanamine
Production of some herbicides and pesticides including atrazine, bentazon, glyphosate; agent for plastics; intermediate in organic synthesis of coating materials, pesticides, plastics, rubber chemicals,
pharmaceuticals and others; additive in the petroleum industry 3-Aminopyridine Synthesis of organic ligand 3-pyridylnicotinamide.
3-Buten-2-one, 3-methyl-, dimethylhydrazone
-3-Cyclohexene-1-carboxaldehyde, 4-methyl-
-4-Fluorohistamine Organic syntheses
Acetic acid
Production of cellulose acetate for photographic film, polyvinyl acetate for wood glue, and synthetic fibers and fabrics; descaling agent, used in
the food industry, in biochemistry Benzoic acid, 2,4-dihydroxy-, (3-diethylamino-1- methyl)propyl ester
-Dimethylamine Dehairing agent in tanning, in dyes, in rubber accelerators, in soaps and cleaning compounds; agricultural fungicide
dl-Alanine Food and pharmaceutical industry; plating chemicals and animal feed Formic acid phenyl ester Used for palladium-catalyzed carbonylation of aryl, alkenyl and allylhalides; used as a reagent for the formulation of amines Hydrogen chloride Used in cleaning, pickling, electroplating metals, tanning leather, andrefining and as an agent for producing a wide variety of products
Methylpent-4-enylamine
-Phenethylamine, p-methoxy-.alpha.-methyl-, (.+/-.)- -Phenol
Precursor to many materials and useful compounds; used to synthesize plastics and related materials; production of polycarbonates, epoxies, Bakelite, nylon, detergents, herbicides such as phenoxy herbicides, and
numerous pharmaceutical drugs.
Phenol, 4-methyl- Production of antioxidants, e.g., butylated hydroxytoluene
Pyrazole, 1-methyl-4-nitro-
-Tetrahydro-4H-pyran-4-ol
-Citric acid can be used as catalyst for the reactions in the HTC treatment, with the effect of increasing the COD content in process waters [82]. Furthermore, CaO, employed as additive for CO2absorber
via carbonation in near-critical water, has been reported to affect process waters composition [59]. CaO created alkaline hydrothermal conditions which promoted solubility of organic compounds, increasing TOC and COD concentrations in process waters from 18,630 up to 55,800 mgL−1, respectively. However, since the gasification process was also increased, TOC and COD dropped to 16,610 and 49,400 g/L, respectively.
HTC process waters from digested agricultural residues are characterized by high values of TOC concentrations. Oliveira et al. [26], analyzing process liquors from dewatered agro-industrial digestate and a combination of it with other biomasses (e.g., corn silage, poultry manure, a mixture of straw and manure, dry straw, cabbage and dough), reported TOC concentrations in the range of 13–26 g L−1.
Similarly, wheat straw digestate leads to a TOC concentration between 5500 and 9500 mg L−1after HTC under the applied conditions (190–270◦C, 6 h) [80]. Similarly to ADSS, wheat straw digestate displayed a significant increase of the TOC concentration between 190–230◦C, while a decreasing tendency was observed between 250 and 270◦C [80].
The concentration of volatile fatty acids (VFAs) in process waters also increased with temperature. Aragón-briceño et al. [34], in their study, reported VFA concentrations of 191, 406 and 715 mg L−1for
160◦C, 220◦C and 250◦C treatments, respectively. Acetic acid, present as a product of hydrolysis, was the main constituent of the VFAs produced in all the treatments. The same has been reported by Wirth et al. [89]. They observed that the dominating organic compound was acetic acid with a concentration of 2.06 g L−1, accounting for 6.2% of TOC of process waters. Propionic acid and butyric acid showed concentrations of 0.12 g L−1 and 0.03 g L−1, respectively. Similar results have been reported by Yu et al. [90]. Higher percentages of VFAs have been measured in the HTC process waters obtained treating digested agricultural biomasses. Treating wheat straw digestate at several HTC conditions (190–270◦C, 6 h), acetic acid was reported to remain largely constant over the process, ranging between 1000 mgL−1 (190 and 230◦C) and 1200 mgL−1 (250 and 270◦C). Propionic acid demonstrates a significant increase from 70 to 130 mgL−1with increasing process temperatures [80].
The presence of acetic and propionic acids was further reported in the process liquor obtained from HTC (220◦C, 6 h) of corn silage digestate, in concentration of 5260 and 340 mgL−1, respectively.
Finally, HTC causes the release of proteins and carbohydrates in the process waters, however, while their concentrations increased from 120 to 160◦C, they were then reported to substantially decrease till 240◦C; this behavior has been observed both treating AGS [90] and municipal ADSS [81]. Nevertheless, the amount of released proteins and carbohydrates from AGS was much higher than that from ADSS, probably because the AGS had a higher organic matter content (VS content) which mainly consists of proteins and carbohydrates.
5.3. Nutrients
HTC process waters from digestate are characterized by a relatively high nutrient content, nitrogen and phosphorous. However, nutrient content depends on HTC operative conditions and initial feedstock composition, including the presence of metals.
5.3.1. Nitrogen
Depending on the feedstock, HTC at different temperatures affects the distribution of nitrogen in HTC by-products, as a result of hydrolysis of proteins and the ammonium release. N-containing compounds are mainly present in HTC solid and liquid by-products, while N-containing gas is negligible [83]. He et al. [28] found that after HTC treatment of dewatered ADSS, N tended to release into process waters: 40% of N remained in the hydrochar while almost 60% was released into liquid phase. HTC temperature can strongly influence N distribution in HTC by-products. Yu et al. [90] reported that N distribution in the solid decreased significantly, increasing the HTC process temperature from 160◦C to 240◦C. This aspect is very important when hydrochar has to be used as an energy carrier, as the N component in fuels is transformed into NOXemissions during the combustion process.
Concerning N in HTC process waters, it can be present as organic nitrogen and ammonium nitrogen (NH4+-N). Organic-N is the dominant form in the water phase from lower processing temperatures
(<250◦
C) [91]. The effect of HTC processing temperature (160–250◦C) on the nutrient behavior in the ADSS process waters was investigated by Aragón-briceño et al. [34]. They reported the distribution of organic N and NH4+-N in process waters at different levels of process severity: while organic-N is the
dominant form in process water for low processing temperatures (160◦C), the levels of nitrogen in the form of NH4+-N significantly increased in the aqueous phase as process severity increased, from 30%
at 160◦
C, to 40% at 220◦
C, to 45% at 250◦
C, respectively. Similarly, Shen et al. [84] showed that both increasing HTC temperature and treatment time would favor transformation from organic nitrogen to ammonium nitrogen. Performing HTC on ADSS, Yu et al. [81] reported that ammonium concentration increased from 320 to 490 mg N L−1with HTC temperature increasing from 120 to 240◦C. However, the maximum increase in NH4+-N was observed when temperatures higher than 180◦C were applied,
for which a maximum decrease in protein was also measured. He et al. [59] studied the effect of
temperature from 200 to 380◦C on nitrogen content in process waters from ADSS. Total dissolved nitrogen in the process water tended to decrease with elevated temperatures, while ammonium started to increase. The same authors showed that even if a high moisture content could favor hydrolysis,
increasing the moisture content of the initial digestate, the dilution effect dominates the nitrogen evolution in the process waters, thus reducing the concentration of total dissolved nitrogen and ammonia. He et al. [83] confirmed the catalysis role of CaO in promoting hydrolysis or cracking of N-containing compounds, as both NH4+-N and organic N increased substantially in process waters.
Additionally, as the Ca/C molar ratio was raised from 0.05 to 0.2, NH4+-N further increased, reaching
the 83% of total nitrogen in process waters, while organic N decreased. This high NH4+-N content
could benefit the downstream N recovery as struvite or ammonium salts.
The HTC reaction time seems to have a secondary role. Yu et al. [81] detected the highest ammonium concentration in the process waters after only 30 min of the HTC treatment. Longer HTC process times did not increase the N concentration. Rather, an increase in treatment time may lead to the decrease in ammonium concentration in process waters, mainly due to the co-precipitation phenomena and loss of ammonia due to stripping.
Finally, nitrogen distribution also depends on the initial type of digestate treated in the HTC process. Depending on biomass treated in AD systems, different concentrations of ammonium can be reached in digestate and, thus, in HTC process waters. A very high value of total nitrogen, 18,610 mg N L−1, was reported by Ekpo et al. [91] for HTC process waters (250◦C, 1 h) obtained from an agro-industrial digestate, due to the co-digestion with manure. The same authors reported that the level of nitrogen in process waters in the form of NH4+-N was 55%, while that of organic nitrogen
was 45%. Lower concentrations were reported for HTC process watersfrom ADSS. Interestingly, Yu et al. [90] found that nutrients in the smashed AGS tended to release more into the liquid than those in the unbroken AGS, indicating that the granule shape may help to entrap the nutrients into the hydrochar after HTC. This could suggest introducing some disintegration treatments of sludge prior to the HTC process in order to favor N release in process waters, and thus N recovery.
5.3.2. Phosphorus
HTC substantially stabilizes phosphorus (P) during the processes [88], as P is mainly immobilized and retained within the hydrochar. Shi et al. [85] reported the redistribution of P and the major metals closely related to the existing forms of P in process waters and hydrochar after HTC of ADSS. Very small amounts of P, Ca, Al, Fe and Mn were detected to be released into process waters (<0.6%) while the majority of them were accumulated in the hydrochar, and the extent of this kind of accumulation increased with the increase of treatment temperature. Performing HTC on ADSS, Wang et al. [78] reported that P was mostly accumulated in the hydrochar, which agreed with the results found by Heilmann et al. [27], demonstrating that the vast majority of P forms could be recovered from hydrochar, due to precipitation of phosphate salts. On the contrary, in their study, Aragón-briceño et al. [34] reported that P was prevalently in the process waters rather than hydrochar, mainly in the form of inorganic P rather than organic P. The different fates of P during HTC, both extraction and immobilization in hydrochar, depend on HTC conditions, feedstock type and the content of inorganics, such as Al, Ca, Fe and Mg, in the feedstock [88,91]. Shi et al. [85] concluded that P speciation changes during HTC may be collectively controlled by the composition and states of metals with high affinity to phosphate and the thermochemical reactions occurring during the process.
In their recent study, Huang and Tang [88] reported speciation of P both in the untreated digestate and hydrochar. They explained that the P speciation (e.g., chemical state and physical distribution) intrinsically determines P solubility, mobility and bioavailability. Indeed, P can exist in several molecular moieties (e.g., organic phosphates, orthophosphate, phosphonate and polyphosphate) and each moiety can be present in several chemical states (e.g., to be adsorbed on other solid surfaces, complexed with cations, incorporated into mineral phases, or precipitated as P-containing solids). Raw ADSS is dominated by orthophosphate and long-chain polyphosphates [99]. Huang and Tang [88] reported that raw ADSS was characterized by:
• Al-associated P species (AlPO4, 40%); • organic P (phytic acid, 20%);
• Fe/Ca-associated P species (ferrihydrite-adsorbed phosphate 13%, octacalcium phosphate, 16%); • alumina-adsorbed phosphate (11%).
Similarly, Shi et al. [85] reported that ADSS has phosphorus mainly in the form of inorganic P (mainly in the form of apatite, Ca5(PO4)3), while its organic P was very low. Further, they also reported
the presence of Ca (14.84%), followed by Fe (5.34%), Al (0.73%) and Mn (0.04%) in ADSS. The presence of these four metals may strongly influence the behavior of phosphorus during the HTC process.
Hydrolysis, decarboxylation and polymerization in the HTC process are responsible for the hydrolysis of polyphosphate into inorganic orthophosphate and further exposed the intracellular and organic-bound P to metals such as Ca, Fe and Al, which have a higher affinity to phosphate than metals such as Na, K and Mg, and are more abundant than metals such as Cu and Zn [99]. As a consequence, phosphates are more likely associated with Ca, Fe and Al, and can precipitate in the hydrochar as phosphate salts [85,99]. Moreover, the relative abundance and forms of these metals determine the P association stoichiometry and capacity (e.g., Fe is mostly present as hydroxide minerals and bound P as surface-adsorbed form, while Ca forms Ca-phosphate minerals in competition with its carbonate and sulfate mineral phases).
In their study on hydrochar, Huang and Tang, [88] found that Fe-associated P increased with increasing HTC treatment time, mainly in the form of ferrihydrite-adsorbed phosphate (from 13% in the raw digestate to 25–27% in the hydrochar). The abundance of total Al-associated P (AlPO4) decreased
from 40% in the raw digestate to 7–19% in the hydrochar, while that of alumina-adsorbed P increased from 11% to 20–24%. The relative abundance of octacalcium phosphate remained unchanged [88,99]. On the contrary, Shi et al. [85] found in their study that phosphorus was mainly transformed from non-apatite inorganic P (the P fraction associated with oxides and hydroxides of Al, Fe and Mn) into apatite-P (the P fraction associated with Ca), mainly because of the high Ca content in the tested sludge and the basic condition of their experiment, which favored the dissolution of non-apatite inorganic P Equation (1) and the release of PO43− that could combine with Ca to form the relatively stable
Ca-bound P Equation (2). This transformation increased with increasing HTC treatment temperature from 200 to 280◦C, while a sharp decrease occurred in correspondence to the organic P fraction in the hydrochar. AlPO4+ 4OH − ↔ (Al(OH)4) − + PO43− (1)
2PO43−+ 3Ca+↔ Ca3(PO4)2 (2)
The effect of different feedwater pH values on the P transformation in hydrochar during the HTC of the ADSS has been studied by Wang et al. [78]. Their results provide a theoretical basis for selecting appropriate methods and strategies for improving the performance of P recycling and reclamation from similar solid waste materials. An acidic feedwater pH promoted the transformation of apatite phosphorus to non-apatite inorganic phosphorus, and of organic P to inorganic P. On the contrary, the non-apatite inorganic phosphorus tended to be transformed into apatite phosphorus and a small part of the inorganic P was transformed to organic P when the digestate was treated in a basic environment.
Ekpo et al. [91], processing an agro-industrial digestate, showed that the extraction of total phosphorus in process waters reduces with reaction severity. During the HTC process (250◦C, 1 h), approximately 7% of the total P was extracted from digestate in the process waters. The levels of total phosphorus gradually reduced as the reaction severity increased to less than 5%. After HTC at 250◦C organic-P in process waters accounted for about 15% of the total TP concentration, while 85% was in the form of phosphate-P. Similarly, Aragón-briceño et al. [34] reported that HTC at low HTC temperatures favors the extraction of organic-P (a complex fraction of phospholipids, DNA and phosphate monoesters) in process waters, which is then broken down into inorganic P (phosphate-P). In their study, Aragón-briceño et al. [34] observed that the organic phosphorus fraction in the liquid phase, which was initially 64.4% of the total P present in the raw digestate, increases after 30 min of HTC treatment at 160◦
C and 250◦
in P content in the solid fraction from 21.7% in the raw digestate to 19.4 and 11% in the hydrochar after treatment at 160 and 250◦C, respectively. Furthermore, in their study, the concentration of orthophosphate in process waters decreased between 25–32% after HTC, probably due to precipitation of some of the inorganic P with metals such as Al, Ca, Fe and Mg, forming insoluble phosphate in colloidal forms or bound to protein.
Yu et al. [81] reported that P in HTC process waters was mainly orthophosphate, which is the reactive P for struvite precipitation. They observed an increase of both orthophosphate and ammonium from 120 to 200◦C after 30 min of HTC treatment, while a decrease in levels of both nutrients was observed for higher temperatures. The concentrations of orthophosphate and ammonium were found to drop when HTC lasted for 60 min, probably due to the precipitation of P minerals with or without ammonium, and loss of ammonia due to stripping process.
5.4. Heavy Metals
The HTC process influences the migration and transformation behaviors of heavy metals in solid–liquid phases of sludge. Even if the majority of heavy metals will be concentrated into the hydrochar, a certain amount can be found in the HTC process waters, representing an environmental issue [96]. Recently, in their experiments, Xu and Jiang [69] demonstrated that high HTC temperatures (240–300 ◦C) promoted the transition of heavy metals in hydrochar from an exchangeable state (adsorbed on the surface) to residual states. Shi et al. [86] carried out the HTC of ADSS at temperatures of 170, 200 and 280◦C. The amount of heavy metals released into process waters increased with the increase of temperature for Zn, Cd and Pb, while the higher release was obtained at 200◦C for Cr, Ni and Cu. Meanwhile, with the increase of reaction temperature, the total content of heavy metals in hydrochar increased.
Studying the HTC process waters from ADSS, He et al. [59] found that while Cu and Pb concentrations remained almost constant, the concentrations of Cr, Zn, Ni, Ca, Al and Fe decreased as the temperature increased from 200 to 380◦C. In particular, Ni was not detectable for temperatures higher than 260◦C, showing the best immobilization performance and Cr precipitation was enhanced by high temperature. During the HTC process, metals are readily transformed into dissolved ions, and metal sulfidation may take place after cooling down, for example in the form of ZnS [100]. Moreover, dissolved Ca, Al, Fe and P could interact with each other to form more stable structures [59]. Their concentrations decreased with the increase of temperature, together with P concentrations.
The dissolution of heavy metals into process waters from sewage sludge is to be coupled with the dissolution of total phosphorus [86], which could represent a chance for heavy metal adsorption, precipitation and thus immobilization in the hydrochar.
The addition of catalysts could both improve the immobilization of heavy metals in hydrochar, and simultaneously enhance the hydrochar yield [88]. He et al. [59] studied the effect of CaO on metal
concentration in process waters. Under high temperatures, the use of an appropriate amount of CaO in the HTC process has a positive effect on the immobilization of heavy metals and mineral elements for further recovery. For example, as a consequence of CaO addition, OH−concentration increased, thus facilitating the dissolution of AlPO4to release PO43−, which subsequently may react with Ca2+to
precipitate as calcium phosphate (Ca3(PO4)2) [85] and with CaF2to form fluorapatite (Ca5(PO4)3F) [101].
In the complex system under near-critical water conditions, CaO tends to go through solid–solid reactions with other components (e.g., silicon (Si) and aluminum (Al)) in the ash of the ADSS [102]. Thus, complex inorganic compounds, such as Ca12Al14O33 (mayenite), Ca(Al,Si)2O4(yoshiokaite),
CaFe3SiO8OH (ilvaite) [101], Ca8Al4(OH)24(CO3)(Cl) (H2O)9.6(hydrocalumite), Ca3(Al,-Fe)2(Si,P)3O12
(garnet), (Ca,Na)2(Mg,Fe,Si,Al)3O7(gehlenite) [103], could be formed. Nevertheless, Xu and Jiang [69]
showed that the addition of FeCl3or Al(OH)3as catalysts had a negative impact on the immobilization
of heavy metals, as it resulted in a significant increase in the exchangeable states of heavy metal in hydrochar rather than residual state.
